Scru bing apparatus

ABSTRACT

An apparatus for scrubbing a stream of air or other gas with a scrubbing liquid (e.g., water). The scrubbing is carried out in successive stages simultaneously with classification of the removed material, the removed material being withdrawn as separate fractions. Each stage is provided with means for scrubbing the gas with liquid droplets and by impingement with surfaces wet with liquid, and also with means for eliminating droplets whereby they are not delivered to the next scrubbing stage.

ite tates atent 1191 i are 1 SCRUBBIING APPARATUS [75] Inventor: ErnestMare, Johannesburg,

Transvaal, South Africa [73] Assignee: ,1. Ronald Hershberger, PaloAlto,

Calif.

[22] Filed: Feb. 22, 1971 [21] Appl. No.2 117,348

[ 1 Jan. 15, 1974 Primary ExaminerRobert Halper Att0meyFlehr, l-lohbach,Test, Albritton & Herbert [57] ABSTRACT An apparatus for scrubbing astream of air or other gas with a scrubbing liquid (e.g., water). Thescrubbing is carried out in successive stages simultaneously ReferencesCited with classification of the removed material, the re- UNITED STATESPATENTS moved material being withdrawn as separate fractions. 885 1854/1908 Serrell 55/227 Each Stage is Pmvided with means for Scrubbing the9073634 12/1908 Novowetz 55/240 gas with liquid droplets and yimpingement with 1,117,309 11/1914 Bentz 55/228 X faces wet with liquid,and also with means for elimi- 1,222,541 4/1917 Donham 55/94 natingdroplets whereby they are not delivered to the 1,223,082 4/1917Lissauer.... 55/227 next scrubbing stage 1,625,831 4/1927 Stark 55/903,036,417 5/1962 Mare 55/90 X 8 Claims, 7 Drawing Flgures 2 B C 12 A 11i 1 ,1 I

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INVENTOR. ERNEST MARE ,dmzjldf %W ATTORNEYS 1 SCRUIBBING APPARATUSBACKGROUND OF THE INVENTION This invention relates generally toscrubbing apparatus of the type in which gas is subjected to thescrubbing action of water or other scrubbing liquid.

Scrubbers are commonly employed for removing various materials orcontaminants from air or other gases. Typical apparatus consists of ahousing equipped with nozzles which discharge sprays of the scrubbingliquid against one or more screens or grids. When a stream of the gas tobe cleaned is directed through the housing, the gas passes through thewater sprays and screens or grids, with materials like dust particlesbeing wet by contact with water droplets and by impingement of the solidparticles on films of liquid on the screen or grid. Water dropletsentrained in the airflow are removed by a so-called eliminator, whichmay consist of baffles arranged to provide tortuous paths for the flow.The removed contaminants are withdrawn with the water which collects inthe lower part of the housing.

The construction of the screens and grids as well as the control ofsprayslargely determines the effectiveness of the scrubbing operations.It has been found that a series of screens is most effective, butdifficulty is often encountered in obtaining the elimination of fineparticles in the arrangements at present in use.

Further known scrubbers frequently produce a slurry of the removedmaterial which is not usable unless further processed. The removedsolids of the slurry will vary according to the source of contaminatedair or gas, but in typical instances will include relatively insolublesolid particles of varying sizes, densities and wettability, solids thatare relatively soluble and which appear in the removed slurry indissolved form, solids that are partially soluble and which appear inthe slurry partly in solution and partly as solid particles, andcontaminating gases or vapors, which are readily soluble or absorbablein water or other scrubbing liquid and which are in the slurry indissolved or absorbed form. Processing of such slurry may includeclassification to separate the undissolved solids into differentfractions, and dewatering for the removal of dissolved solids in aneffluent.

SUMMARY OF THE INVENTION An object of the invention is to provide aapparatus which will facilitate the removal of small particles of minusmicrons in size and which may also result in classification and removalof material from the gas stream in separate fractions.

Another object is to provide an apparatus of the above character whichwill have effective scrubbing action and which makes use of novelslotted plates against which the spray is directed and through which thegas is caused to flow.

In general the removal of small particles or classification and removalof material from the gas stream is carried out by passing a gas flowthrough a plurality of screens in a gas flow passage while sprayingliquid onto said screens wherein at least one screen following the firstin the series is formed as a plate with a plurality of punched out stripportions offset with respect to the plane of the plate to provide slitsbetween the plane of the plate and the strip portions and removing thesmaller particles from the gas stream at this screen and spray assembly.

Also, before the gas from one screening stage is delivered to the nextsuccessive stage, entrained droplets are eliminated. The liquidsdraining from each of the stages are separately collected, therebyproviding different classified fractions of the material removed fromthe gas.

The apparatus comprises a gas flow passage, a plurality of screensarranged in series along the gas flow passage, each screen having liquidsprays associated therewith to form an assembly and at least one screenfollowing the first of the series in the downstream direction formed asa plate with a plurality of punched-out strip portions offset withrespect 'to the plane of the plate and providing slits between the planeof the plate and the offset strip portions and with this latter screenand spray assembly adapted for finer screening than any assemblypreceding it in the series.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side elevation in sectionshowing apparatus incorporating the present invention.

FIG. 2 is a cross-sectional view taken along the line 2 2 of FIG. 1.

FIG. 3 is an enlarged detail showing a portion of one of the slottedimpingement plates.

FIG. 4 is a detail in section taken along the line 4 4 of FIG. 3.

FIG. 5 is a detail in section taken along the line 5 5 of FIG. 4.

FIG. 6 is a detail in section like FIG. 5 but indicating flow in areverse direction.

FIG. 7 is a detail on an enlarged scale in section showing one type ofeliminator which can be used for each of the stages.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus shown in thedrawing consists of a housing 10 formed of the connected top and sidewalls 11 and 12. The inlet opening 13 at one end of the housing connectswith a source of gas to be cleaned or scrubbed, with the gas moving atvelocities which may be within the range of about 4 to 10 ft. persecond. In some instances the gas may be delivered by a suitable bloweror fan, while in other instances there may be a direct conduitconnection to an industrial processing step which is delivering the gasat a relatively uniform and suitable velocity. The outlet opening 14 atthe other end of the housing may deliver the scrubbed gas to theatmosphere, or to an industrial processing step. Within the housing aplurality of scrubbing stages A, B And C are established, with the gasbeing passed successively through these stages. Below each stage thelower walls of the housing are made to provide liquid collecting troughs16a, 16b and 160. These troughs connect with drainage pipe 17a, 17b and170.

The parts provided in each scrubbing stage include the spray pipes 18that are provided with spray nozzles 19 and which connect with a commonheader pipe 21 extending to the exterior of the housing. As desired, thespray nozzles and pipes for each scrubbing stage A, B and C may beconnected independently with different sources of liquid, depending uponthe particular constitutents to be removed. For example, water can besprayed in one stage to remove particulates and caustic can be sprayedin another stage to remove acid fumes.

Other suitable liquids known to the art may also be utilized.

The pressure at the spray nozzles 19 may be varied between stages,depending upon particular specifications and requirements. Thus, thenozzles for the downstream stages B and C may operate with progressivelylower spray pressures than the stage A nozzles, or the reverse ordercould be used.

On the downstream side of the pipes 18 there is a screen or plate 22which forms impingement means and which extends at right angle to thegeneral direction of flow. As will be presently explained, each plate 22is provided with a series of slots through which the gas flows withabrupt change in direction. On the downstream side of each plate 22there is an eliminator 23 for the purpose of removing entrained liquiddroplets from the gas flow. This eliminator may be of a type commonlyused in connection with gas scrubbers and which may provide a pluralityof nested zigzag plates or baffles through which the gas flows.

The preferred construction of each impingement screen or plate 22 isillustrated in FIGS. 3 6. It consists of a flat metal plate providedwith a plurality of struck-out metal strip portions 26. As is evidentfrom FIG. 4 each struck-out strip 26 provides an impact surfaceextending parallel to the main body of the plate 22 but offset from theplane of the same. By virtue of the offsetting, there is a rectilinearslot 27 formed along each edge of each strip portion 26, and the slotsare each in a plane at right angles to the plane of the plate 22. Theoffset portions 16 may be on the downstream side of the plate 22 asshown in FIG. 5, or they may be upon the upstream side as shown in FIG.6. The choice in this connection depends upon the character of thematerial in the gas which is to be removed.

While the preferred embodiment illustrates apparatus in which all threestages include the heretofore described impingement plates 22, theinvention contemplates that one or more of the plates 22 may be used incooperation with other gas scrubbing screens of a type known in the art.For example, an impingement plate 22 for fine screening would preferablybe positioned downstream of one or more primary scrubbers or screens ofthe type disclosed in U.S. Pat. No. 3,036,417, e.g., a screen defined bya pattern of rodlike elements.

As previously mentioned, the eliminator 23 for each stage may beconstructed to provide zigzag paths for flow of the gas. Such aneliminator is indicated in FIG. 7. The series of plates 28 are bent andnested together to form the zigzag paths 29. When a gas having aconsiderable amount of liquid in the form of entrained droplets ispassed through such an eliminator, the droplets are caused to impingeupon and wet the surface of the plates 28, whereby entrained liquid iseffectively removed from the gas. The liquid thus deposited upon thesurfaces of the plates 28 drains downwardly.

Operation of the apparatus described above and the carrying out of themethod which includes classifying constitutents from a gas stream are asfollows. The gas flow to-be cleaned is delivered at a relatively uniformvelocity to the inlet opening 13, and caused to flow through the housing10. In the first stage A, the gas'is subjected to the scrubbing actionof the scrubbing liquid discharged from the spray nozzles 19. Thescrubbing action is due not only to contact of spray particles with thegas, but also to the wetting of the upstream surfaces of the plate 22and struck-out strip portions 26, and the continual impingement of thegas against these surfaces. When the spray particles are discharged fromthe nozzles with sufficient velocity, there is some shattering andrebound of atomized droplets upon impact with the plate 22, and thisaids in obtaining contact of material carried by the gas with theliquid. Gas passing through the plate 22 flows through the slotlikeopenings 27, which necessarily requires an abrupt change in the flowdirection, as will be apparent from FIGS. 5 or 6. This abrupt changeaids in effective removal of material from the gas. The gas passagescreated by the slot-like openings are also effective in creating flowturbulence for increased wetting of the solids. Thus, the strips 26 arealigned in side-by-side relationship so that the flow paths issuing fromthe confronting openings 27 of any adjacent pair of the strips for theplate of FIG. 5 converge together and turbulently intermix. Similarly,the flow paths entering the openings 27 of each strip for the plate ofFIG. 6 converge together and intermix. After the gas has passed throughthe plate 22, it carries a substantial amount of liquid in the form ofentrained liquid droplets. Such liquid droplets are removed by flow ofthe gas through the eliminator 23.

Liquid which drains downwardly from the stage A is collected in thetrough 16a. This liquid is formed by collected spray particles, byliquid flowing downwardly in the form of films on the plate 22 and theportions 26 of the plate, and by liquid flowing downwardly over thesurfaces of the baffle elements of eliminator 23.

In flowing downwardly over the wetted impingement surfaces, the liquidaccumulates material removed from the gas, and this liquid together withany liquid which falls directly from the sprays into the collectingtrough 16a contains materials which have been removed in this stage. Thegas then continues to flow through the second scrubbing stage B, wherethe process described above is repeated. Here again the gas is subjectedto the scrubbing liquid, and some of the remaining material contained inthe gas is removed and appears in the liquid collected in trough 16b. Inscrubbing stage C the process is again repeated, and here again residualmaterial contained in the gas is removed and appears in the liquidcollected in trough 16c.

Assuming that the gas carries a variety of contaminants or othermaterials, there will be selective removal of material in the stages A,B and C, or in other words, there is a classifying action whereby thematerial in the collected liquid in troughs 16a, 16b and will differ.For example, if the gas carries relatively coarse and relatively finedust solids, the greater proportion of the coarse solids will tend to beremoved in the stage A, whereas the finer dust particles will tend topass on and be removed in the stages B and C. Also in an instance wherethe solid particles carried by the gas consist of particles which arereadily wettable, and particles which are not as wettable, the morewettable particles will tend to be removed in the collecting trough 16ain preference to the particles which are not readily wettable, and thelatter will then tend to be removed in succeeding stages B and C.

In instances where the gas may contain gaseous contaminants such asoxides of sulphur, the first stage is generally most effective in theirremoval, and they will appear mainly in the material collected in trough16a. Remaining amounts of such contaminants will be removed in thestages B and C.

As previously stated, the slotted plates 22 are highly effective inpromoting effective scrubbing action. The impacting surfaces presentedto the spray particles are all at right angles to the general directionof flow through the housing. As previously mentioned, there is an abruptchange in flow direction as the gas flows through the slots 27. Thewidth of these slots may be selected in accordance with the size of thesolids carried by the gas. In general, the slots should have a width atleast three times the diameter of the largest particles carried by thegas. In some instances, it may be desirable for the plates of thesuccessive stages to differ with respect to the width of the slots 27.

The gas being scrubbed may contain solid particles that are soluble inthe scrubbing liquid. In such instances, a part or all of such solidsmay be dissolved, and such solids will then appear in dissolved form inthe effluent collected in troughs 16a, 16b and 16c. A highly solublematerial will be largely dissolved in stage A, and will thus appearmainly in the effluent collected in trough 16a.

Apparatus of the type described above will also find application and usein a scrubbing method which does not include clsssifying of theconstitutents separated from the gas stream. A method of this characterwould include delivering the gas flow at a relatively uniform velocityto the inlet opening 13 and causing it to flow through the housing atvelocities between 4 10 fps. In the separate stages the gas is subjectedto the scrub bing action of the scrubbing liquids discharged from thespray nozzles in the three stages. The scrubbing action in each stage issimilar to that described above in that the removal of material from thegas results both from contact of spray particles with the gas and fromthe wetting of upstream surfaces of the plates 22 and struck-out strips26, and the continual impingement of the gas against these surfaces.

In the method where classification is not required the gas may carryonly one, or a plurality, of contaminants or materials to be separated.These materials are of a character such that they will be separated andcollected in any of the three stages. The liquid and entrained materialdrains downwardly in stage A and carries with it the materials separatedfrom the gas in that stage for collection in trough 16a. The gas thencontinues to flow into the next scrubbing stage B where the scrubbingprocess is repeated. The scrubbing liquid removes additional materialfrom the gas and drains downwardly where it is collected in trough 16b.The gas continues into stage C where the process is again repeated withresidual material removed from the gas by liquid which collects intrough 160.

An example of the method for scrubbing a gas where classification of theconstitutents is not required is as follows: Apparatus of the typedescribed was used for scrubbing the water-saturated airstream producedfrom a reactor vent system in a complex fertilizer plant. The streamcontained ultra-fine ammonium nitrate with minute quantities of ammoniaand nitric oxides. The scrubbing apparatus comprised three scrubbingstages with dimensions of 6 feet X 5 feet in cross section and overalllength of 16 foot -5 inches. Each stage included two impingement plates22 having dimensions of 3 feet X 5 feet. Each impingement plate wasformed with a plurality of struck-out strips 2 Aiinches long X Ainchwide offset downstream of the plane of the plates 0.038 inches. Theimpingement plates were spaced 3 foot -7 inch apart along the length ofthe scrubber apparatus. The spray apparatus for each stage included 72nozzles spaced 17 inches upstream of respective impingement plates.Ammonium nitrate scrubbing liquid was 5 sprayed through the nozzlesunder a pressure of approximately 200 psig. Eliminators 23 havingdimensions of9 27/64 inches wide X 5 -foot 1 Vs inches high werepositioned in the flow path 7 inches downstream of respective plates 22.

The scrubber inlet gas flowed at a rate of 5000 ACFM, a temperature of180 F, a density of 0.0706 lbs/ft. and a particulate loading of 2.2grain/DSCF. The pressure drop across the scrubber apparatus was measuredin two inches of water. The scrubber outlet gas flowed at a rate of 5000ACFM, a temperature of on particulate matter of varying size:

Particle Collection Size Efficiency 0.5 microns I007:

0.5 0.25 microns 86% 0.25 microns 77% Average 97.5%

No detectable difference in either outlet fume loadings or stackopacities were noted operating with fresh water or recycled liquorcontaining up to percent NH NO concentrations.

The foregoing illustrates that the invention is capable 'of collectingand removing relatively small particles with very high efficiency ascompared to known scrubbing apparatus and methods.

I claim: I

1. Gas cleaning apparatus comprising a gas flow passage, a plurality ofscreens arranged in series along the gas flow passage, each screenhaving liquid sprays associated therewith to form an assembly with theliquid sprays being directed toward respective screens on the sidesthereof upstream of the gas flow, at least one screen following thefirst of the series in the downstream direction formed as a plate with aplurality of punched-out strip portions offset with respect to the planeof the plate and with the major area of the punched-out strips parallelto the plane of the plate to provide openings on each side of the stripportions transverse to the gas flow passage, with this latter screen andspray assembly being adapted for finer screening than any assemblypreceding it in the series.

2. Gas cleaning apparatus as claimed in claim 1 in which the stripportions are offset towards the upstream side of the gas flow passage.

3. Gas cleaning apparatus as claimed in claim 1 in which the stripportions are offset towards the downstream side of the gas flow passage.

4. Gas cleaning apparatus as claimed in claim 1 in which screens oneither side of the screen defined are made of rod-like elements.

5. Gas cleaning apparatus as claimed in claim 4 in which sprays areadapted for use with different liquid pressures for each screen.

each screen.

8. Gas cleaning apparatus as claimed in claim 4 in which moistureeliminators are located in the gas flow passage after each screen.

1. Gas cleaning apparatus comprising a gas flow passage, a plurality ofscreens arranged in series along the gas flow passage, each screenhaving liquid sprays associated therewith to form an assembly with theliquid sprays being directed toward respective screens on the sidesthereof upstream of the gas flow, at least one screen following thefirst of the series in the downstream direction formed as a plate with aplurality of punched-out strip portions offset with respect to the planeof the plate and with the major area of the punched-out strips parallelto the plane of the plate to provide openings on each side of the stripportions transverse to the gas flow passage, with this latter screen andspray assembly being adapted for finer screening than any assemblypreceding it in the series.
 2. Gas cleaning apparatus as claimed inclaim 1 in which the strip portions are offset towards the upstream sideof the gas flow passage.
 3. Gas cleaning apparatus as claimed in claim 1in which the strip portions are offset towards the downstream side ofthe gas flow passage.
 4. Gas cleaning apparatus as claimed in claim 1 inwhich screens on either side of the screen defined are made of rod-likeelements.
 5. Gas cleaning apparatus as claimed in claim 4 in whichsprays are adapted for use with different liquid pressures for eachscreen.
 6. Gas cleaning apparatus as claimed in claim 4 in which spraysare adapted for use with different liquids for each screen.
 7. Gascleaning apparatus as claimed in claim 4 in which separate collectiontroughs are associated with each screen.
 8. Gas cleaning apparatus asclaimed in claim 4 in which moisture eliminators are located in the gasflow passage after each screen.